Heat and humidity characterize the San Antonio summer months, with colder temperatures in the winter. The temperature rises to 90°F. or above on an average of 116 days per year. Temperatures of 32°F or below occur on an average of 21 days per year, often for only an hour or so in the early morning.
Humidity averages about 80% in the early morning during most of the year, but drops to about 50% in the late afternoon. In San Antonio, the sun shines 50% of the daytime in the winter and more than 70% of the daytime in the summer.
Rainfall is fairly well dispersed throughout the year. Thunderstorms occur during the late spring and throughout the summer. Precipitation during winter usually occurs as light rain or drizzle, and snowfall is rare. Heavy fog occurs mostly in the winter months. Because San Antonio is only 140 miles from the Gulf of Mexico, the area is affected by tropical storms with heavy rainfall and the potential for tornadoes. Storms can cause flash-flooding in many areas, and extended rainfall can occasionally cause very severe flooding in areas along river channels.
Park visitors need to heed warnings concerning heat stress during the summer by drinking plenty of water and staying out of the direct sun. And during the rainy seasons, do not drive through moving water at low water crossings, many of which will be barricaded to prevent drivers from crossing.